Disclosed herein is an apparatus and method that applies release fluid to a leveler in a printing apparatus.
Presently, wax based phase change inks, otherwise known as solid inks, and radiation curable gel inks, such as ultraviolet inks, can be jetted directly onto a media web or cut sheet media in printing devices using ink jet direct marking technology. For solid inks, after ink has been deposited on a media sheet, the ink must be leveled by a leveler. The leveler includes a potentially heated leveler roll which holds a sacrificial release fluid that in turn contacts the ink on the media. A backing pressure roll supplies nip pressure in order to spread the ink. In order to prevent ink on the media sheets from offsetting to the leveler roll, the leveler roll must maintain a uniform layer of a sacrificial release fluid, usually a silicone oil, to avoid intimate contact between the ink and bare leveling surface. In the case of radiation curable inks, a sacrificial release fluid is also used, but unlike the case of solid ink where a more viscous oil is used, radiation curable inks have been shown to perform well with water based fluids containing small amounts of polymers and/or surfactants along with other various additives. The resulting viscosity of the release fluids used with radiation curable inks is substantially less than the fluids typically used with solid ink and are in fact close to the viscosity of water.
Contact leveling may be an integrated part of an ultraviolet gel ink printing system. As discussed, the contact leveler is essentially a pair of rollers, much like a fuser roll/pressure roll configuration used in xerographic processes, which is required to spread the ink prior to final radiation cure. Much like a fuser, the leveling roll needs to have surface characteristics that inhibit the ink from adhering to, and remaining at least partially adhered to the leveling roll as the media leaves the nip. This process of having cohesive failure within the ink and leaving potions of the image behind and adhered to the leveling roll is typically referred to as offsetting.
Since the radiation curable ink typically is using water based released fluids, an analogy can be made to fountain solutions that are used in lithographic printing processes In lithographic printing, a device is used to deliver a controlled and metered amount of fluid from a sump, through the roll train where the film is continually split and eventually to the imaging plate surface in a uniform layer. This roll train, commonly referred to as a dampening system, is both expensive and requires a high degree of skill to setup and maintain. Furthermore, it is also difficult to adjust fluid delivery rates while running the damping system. This is an issue in radiation curable inks, because there is a need to account for desired changes due to media carry-out rates, contamination of rolls, different inks, different ink amounts related to digital imaging, etc. Unlike the lithographic process, radiation curable leveling requires a uniform layer across the entire roll to avoid intimate contact between the ink and the entire leveling surface. Digital printing by nature has shorter run lengths and therefore setup times become more relevant. Furthermore, newspaper machines use a roll train that is sprayed with a water solution. However, these machines continuously apply the solution to the roll train, which would apply excessive fluid for other applications.
Additionally, in cut sheet operations some damping systems or related release agent management systems start out with a large amount of film on the rolls, which can lead to the first few sheets having too much release agent resulting in inconsistent output across the media run. Again, since digital print runs are short, this becomes a quality issue as well.
Thus, there is a need for an improved apparatus and method that applies release fluid to a leveler in a printing apparatus.